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Where do we define the "surface" of a gas planet?

Astronomy Asked by Carlos Vázquez Monzón on September 28, 2021

Since gas giant consist of most gas components, where do we establish their "surface"?

My take is basically to take the limit in which all light is opaque. For example, in this photo:

Photo of Jupiter. Credit to NASA, ESA, A. Simon, and M.H. Wong

The surface, then, will be the limit of the black blackground with the planet.

Any other way to formally define the "surface" of a gas giant?

2 Answers

There are two common definitions in use for the surface of gas planets:

  • The 1-bar surface: As pressure increases, the deeper in we go into the gas planet, we will hit a pressure of 1 bar at some altitude. Gas at this altitudes will usually sit deep enough in the gravitational well and be of a near-uniform density and temperature, as to not be influenced by exterior parameters, for example the solar wind. Therefore, the altitude of the 1-bar level will remain essentially constant, for short astronomical times.
  • The $tau=2/3$-surface: This is the altitude, from which photons can escape freely into space. This happens at an average optical depth $tau$ of 2/3. It is essentially what you see in your image as the limit of the black background. For the sun one end of the photosphere is the average $tau=2/3$-surface, and for transiting exoplanets this is identical to the measured transit radius at that wavelength.

There is no hard relation between those two surfaces, but in general their altitude will not be different by more than a scale height, as at around 0.1-1 bar the gaseous atomic and molecular bands become enormously pressure broadened, which makes the atmosphere quickly opaque at most wavelengths, for the usual gas giant components.

Correct answer by AtmosphericPrisonEscape on September 28, 2021


Any other way to formally define the "surface" of a gas giant?


Other than what Michael and AtmosphericPrisonEscape are proposing, you could also set the surface at the actual surface of Jupiter's metallic liquid core. The four gas giants (except Saturn perhaps which may be fully gaseous) have solid or liquid cores that have an actual surface. Jupiter's liquid core is about as big as the Earth and has about 10 Earth masses.

Answered by Ioannes on September 28, 2021

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